National Radio Astronomy Observatory

Detecting Extrasolar Planets

With Millimeter-Wave Observatories

Do nearby stars have planetary systems like our own? How do such
systems evolve? How common are such systems? Proposed radio
observatories operating at millimeter wavelengths could start
answering these questions within the next 6-10 years, according to
scientists at the National Radio Astronomy Observatory (NRAO).
Bryan Butler, Robert Brown, Richard Simon, Al Wootten and Darrel
Emerson, all of NRAO, presented their findings today to the
American Astronomical Society meeting in San Antonio, TX.

Detecting planets circling other stars is a particularly
difficult task, and only a few such planets have been discovered
so far. In order to answer fundamental questions about planetary
systems and their origin, scientists need to find and study many
more extrasolar planets. According to the NRAO scientists,
millimeter-wavelength observatories could provide valuable
information about extrasolar planetary systems at all stages of
their evolution.

"With instruments planned by 2005, we could detect planets the
size of Jupiter around a solar-type star out to a distance of 100
light-years," said Robert Brown, Associate Director of NRAO. "That
means," he added, "that we could survey approximately 2,000 stars
of different types to learn if they have planets this size."

Millimeter waves occupy the portion of the electromagnetic
spectrum between radio microwaves and infrared waves. Telescopes
for observing at millimeter wavelengths utilize advanced
electronic equipment similar to that used in radio telescopes
observing at longer wavelengths.

Millimeter-wave observatories offer a number of advantages in
the search for extrasolar planets. Planned multi-antenna
millimeter-wave telescopes can provide much higher resolving
power, or ability to see fine detail, than current optical or
infrared telescopes. Millimeter-wave observations would not be
degraded by interference from the "zodiacal light" reflected by
interplanetary dust, either in the extrasolar system or our own
solar system. Another important advantage is that, at millimeter
wavelengths, the star's brightness poses less of a problem for
observers because, while it is still brighter than a planet, the
difference in brightness between the two is far less. Because of
the physical nature of the objects themselves, protoplanets in
different stages of formation could readily be detected by advanced
millimeter-wave observatories.

The observatories that could provide these advantages are the
Millimeter Array (MMA), a proposed 40-antenna millimeter-wave
telescope that could be operational by 2005, and an upgraded
version of the existing Very Large Array (VLA), a 27-antenna radio
telescope in New Mexico.

The MMA is a radio telescope designed to operate at wavelengths
from 11.5 millimeters down to 0.5 millimeters, or frequencies from
26 to 650 GHz. It will use 40 precision antennas, each 8 meters in
diameter, all operating in concert to produce extremely high-
resolution images. As is done with the existing VLA and VLBA radio
telescopes, the signals from all the MMA antennas will be processed
in a special-purpose computer called a correlator. The processing
of the signals corrects for atmospheric propagation effects and for
the fact that the "synthesized telescope" is in fact made up of
individual antennas.

Planning for the MMA began as early as 1983, and a number of
scientific workshops have allowed U.S. researchers to make known
their needs for a millimeter-wave observatory to serve a wide
variety of specialties. The National Science Foundation (NSF)
provided initial design funding to NRAO in 1995 for MMA studies.
Currently, MMA efforts are centered on selecting an appropriate
site, which must be very high, dry and flat. A site at 16,500 feet
elevation in northern Chile is now being tested. Hawaii's Mauna Kea
is also under consideration. If funding is approved for the MMA,
the instrument could be in operation by the year 2005. The MMA is
expected to be an international instrument, with funding from both
U.S. and foreign sources.

The MMA will be capable of imaging planetary systems in the
earliest stages of their formation. The MMA will be able to detect
many more young, low-mass stellar systems and to examine them to
determine if they have the disks from which planetary systems are
formed. In addition, the MMA could be used to examine the
properties of these disks in detail. The properties that could be
examined include size, temperature, dust density and chemistry.

A number of enhancements have been proposed to the MMA,
including longer baselines for greater resolution, the ability to
observe at higher frequencies, and greater signal bandwidth. This
enhanced MMA would have the sensitivity to directly detect very
young giant planets in the nearest star-forming regions, the
resolving power to distinguish them from their central stars, and
the ability to detect giant planets by measuring their
gravitational effect upon their parent stars and thus determine
their masses.

The VLA, dedicated in 1980, also could contribute to the search
for extrasolar planets if proposed upgrades are implemented. Though
originally designed to operate at a highest frequency of 24 GHz,
the VLA recently has been equipped with receivers for 40-50 GHz.
Funding for receivers in this range, at a wavelength of 7
millimeters, was provided in 1993 by the government of Mexico. The
VLA now has 13 of its 27 antennas equipped with these 40-50 GHz
receivers. Plans for upgrading the VLA include equipping all
remaining antennas with such receivers, improving its electronics,
and improving its resolution by adding antennas at extended
distances.

The upgraded VLA will be able to study the inner parts of the
dust disks surrounding young stars -- disks that are believed to
be the precursors to planetary systems. The inner parts of such
disks are obscured at shorter wavelengths. The enhanced VLA will
be able to reveal processes occurring in these disks at scales
comparable to the size of our own Solar System.

"The reason we hope to search for extrasolar planets with
millimeter-wave telescopes is that we can build on the experience
U.S. astronomers have gained with both millimeter observing and
aperture-synthesis telescopes such as the VLA over the past two or
three decades," said Brown. He added, "We look forward to applying
this expertise to the challenge of answering one of mankind's
oldest questions."